A time resolved fluorescence and quantum chemical study of the solar cell sensitizer D149

Abstract

The steady-state spectroscopic properties and ultrafast relaxation dynamics of the very promising indoline-based sensitizer dye D149 are reported in various organic solvents. For the study of relaxation dynamics of both S2 and S1 states, time resolved femtosecond upconversion spectroscopy was used. Quantum chemical calculations of excitation energies and dipole moments at the DFT level as well as using high-level correlated second-order approximate coupled-cluster model with singles and doubles (CC2) have been also performed. The difference between the first excited and ground state dipole moments was found experimentally to be ∼13D in agreement with calculations. According to our time resolved results, the population of the S2 state has shown a rapid internal conversion to S1, taking place within 0.15–0.45 ps, depending on the solvent. This is followed by a slower decay component indicating that some S2 species relax on a long timescale. As the detection is shifted to the transition from the S1 state, the overall decay dynamics is retarded, while a slow fluorescence rise is observed at long wavelengths, indicating a time-dependent spectral shift. The lifetime of S1 was found to depend on the solvent used, being 630 ps in toluene, 720 ps in chloroform, 670 ps in THF, 700 ps in dichloromethane, 220 ps in acetonitrile and 190 ps in EtOH. The time resolved emission spectra as well as the solvent response function, C(t), obtained in the aprotic solvents, have shown that as the solvent polarity increases a more rapid spectral shift is observed. An exception is found in the case of the protic solvent EtOH due to specific H-bonding effects. The time resolved emission spectra in all solvents experience no significant change of shape and width with time, except from THF, where the initially formed Intramolecular Charge Transfer state is observed at early times.